Baking all paint slots into one material
Yes, just 

1) Click image and create new image with the same size as your paint slots

2) Select the paint slots that you want to bake.

3) Then go to the Properties panel > Render tab.

3a) Go all the way down to the Bake dropdown and select the Bake Mode that you want. e.g. Textures



4) Go to edit mode and select your object

5) If the image you just created is not there anymore bring it back



6) Click Bake.

Don't forget to save your image